There are known passive power factor correction methods that use a network frequency LC resonance circuits operating as a higher frequency harmonic filter. For instance, a higher harmonics filter is connected in series with a bridge rectifier on its AC side (A. R. Prasad, P. D. Ziogas, S. Manias. Passive input current waveshaping method for three-phase diode rectifiers. IEE Proceedings-B, Vol. 139, No. 6, November 1992, pp. 512-520). There is also known a converter with parallel and series resonance alternation (K. Janson, J. Järvik. AC-DC Converter with Parametric Reactive Power Compensation. IEEE Trans. Power Electron, Vol. 46, June 1999, pp. 554-562) where coil and capacitor, through diodes of the rectifier bridge, are forming a parallel or series resonance circuit of network frequency. Both resonance circuits are reducing the higher harmonics in mains current. The advantage of passive power factor correction methods is in their simplicity, but their disadvantage at network frequencies (50/60 Hz) is too large weight, size and cost.
There are also known active power factor correction methods, which use a converter to shape mains current; these converters are operating at the higher frequencies compared to the network voltage frequency and they contain controlled electronic switches (N. Mohan, T. Undeland, W. Robbins. Power electronics. Wiley, N.Y., 1995, pp. 488-494). The reference curve corresponding to RMS value of the converter mains current and similar to the waveform of mains voltage is obtained in the control circuit of the device for mains current shaping. The mains current shaping device is controlled on the basis of measured differences between instantaneous values of the reference and real mains current; there is a boost converter for that purpose. The advantage of the method is a reduced weight and size of coils and capacitors in switch-mode converters because of the higher switching frequency. The disadvantage of the method is that, for power factor correction, an additional converter must be used, which usually cannot be simultaneously used for regulation of the rectified voltage. Another disadvantage is a complicated control circuit that has to include both voltage and current sensors is required.
There are also known indirect active power factor correction methods, which are the most close to present invention and where the shape of mains current is not shaped directly, but the input impedance of the converter is kept constant using controlled modulators. (E. Herbert. Method and apparatus for controlling the input impedance of a power converter. U.S. Pat. No. 5,132,606). In case of constant input impedance the mains current of the converter automatically becomes similar to mains voltage in shape and no distortion occurs in the current waveform. This is an advantage of the method. The disadvantage in implementation of the described method is in a large number of electronic switches of the modulator and complicated control. Input impedance could, in principle, kept constant using matching transformer having fast-changing turns ratio in time domain. (S. Singer. Power conversion and control with zero AC current harmonics by means of a time-variable transformer. IEE Proceedings, Vol. 131, Pt. G, No. 4, August 1984, pp. 147-150). The implementation of such transformer using passive components is not known and drawback of the implementation using active components is in its complexity.
The input impedance of AC/DC converters changes during every half-cycle of mains voltage due to switching processes in the converter. Therefore, the shape of the mains current instantaneous values is distorted and differs significantly from the shape of instantaneous values of mains voltage, resulting also in decreased power factor.
The distortion of current shape distorts the quality of mains voltage. Hereby, additional losses appear both in equipment of mains as in many consumers' devices supplied from the mains. Moreover, ageing of the insulation is speeding up both in grid and in consumer devices.
The goal of the invention is a simple method and simple means for correcting the power factor, reducing the costs of correcting the power factor, and increasing reliability.